- Verify that file holes work (they should, but must be checked).  Add a
 regression test for this feature.

- Fix and complete code marked with `XXX' and `TODO' tags.

- Adjust code style - remove the /* ---- */ markers and use standard
 struct vop_*_args assignment into *ap with comment like:
       struct vop_link_v2_args /* {
               struct vnode *a_dvp;
               struct vnode *a_vp;
               struct componentname *a_cnp;
       } */ *ap = v;

 jmmv: I may agree with the removal of the visual lines, but not with the
 vop_*_args assignment.  Those are very ugly and add a lot of redundancy.